1. Field Of the Invention
This invention relates to a light-emitting diode, such as may be used as a source of optical radiation in a printer or a display, and in particular, to such a source fabricated so as to decrease the effects of a non-uniform distribution of radiation.
2. Description Of The Prior Art
One shortcoming to the use of light-emitting diodes (LEDs) as light sources is the inherent irradiance problems resulting from the configuration of electrical contacts at the LED, as shown in FIG. 1A. LED 12 is a two-terminal semiconductor device characterized primarily by its ability to pass current readily in only one direction. Current flow is achieved by creating an electrical barrier within the semiconductor or at the surface of the semiconductor. Passage of the current is allowed when the electrical barrier is reduced by applying a "forward" voltage, but current is blocked when the electrical barrier is increased by applying a "reverse" voltage. When current passes through, LED 12 emits optical radiation 14.
The first terminal of the LED is connected at the interface of a bottom surface of LED 12 and a first electrode 16. First electrode is bonded to LED 12 by means of a solder or a conductive epoxy 18. Because conductive epoxy 18 is opaque, the light emitted through the bottom surface of LED 12 is blocked.
The second terminal of LED 12 connects to a substrate electrode 22 by a bonding wire 26 that is electrically connected to a pad 24 integrated on a top surface of LED 12. Pad 24 is configured so as to create substantially uniform flow of electricity throughout LED 12 and thus allow the passage of current. Pad 24, however, blocks light 14 emitted from a portion of the top surface of LED 12. An example of an illumination distribution of the emitted light across plane A--A' illustrates the problem as is shown by graph 34 in FIG. 1B.
Further, bonding wire 26 alters the transmission of emitted light 14 from a portion of the top surface of LED 12. Emitted light 14 that is obstructed by the bonding wire 26 along the angle .theta. will have an altered angular illumination distribution as the bonding wire 26 creates a strong shadow and a non-symmetrical, angular light-emission characteristic. FIG. 1C shows the resulting illumination distribution as curve 30 plotted on the ordinate as the magnitude of the illumination against an angle .theta. across the surface of LED 12 in comparison with a uniform distribution 32.
Another drawback of using LEDs as light sources is that bonding wire 26 is often coated with a passivation layer 28 that non-uniformly affects emitted light 14. This is a result of the tendency of passivation layer 28 to clump at places along bonding wire 26. Moreover, LEDs that are edge emitters have a different angular light emission characteristic and contribute to a non-uniform irradiance distribution in any optical system in which they are incorporated.
Accordingly it is an object of this invention to provide a light source which minimnizes the effects of electrical contacts on emission.
It is another object of the invention to minimize the effects of edge emitters on an irradiance distribution.